Various clamping devices are known in the art for providing an electrical connection to a conductor. These include C-clamps and various forms of strap connectors.
These known clamping devices typically have first and second arms that extend at a fixed distance from each other from a base. The arms and base form a C- or U-shape with the arms being positioned adjacent the sides of the conductor. The base can be positioned against or slightly above a surface of the conductor. The clamp can be attached to the conductor through one or more mechanisms, such as an elongated screw, once the clamp is placed on the conductor.
The first and second arms are rigidly fixed on the base with the distance between the arms consequently also being fixed As a result, these clamps can only be used for conductors of a predetermined limited range of widths, generally equal to the length of the adjustable screw. If the conductor has a width outside this limited range of widths, the clamp is simply not attachable to the conductor and would fall off.
Since conductors are not of a standard shape and vary in width, a clamp that is adjustable to the width of the conductor is desirable. This is particularly the case when the conductor is a hollow metal box, such as a utility box for electrical power service to a building. Where the conductor is to be used in an outdoor environment, it is typically painted and/or treated with a coating to prevent rusting and to provide for a longer life. Also, dirt, corrosion and other non-conducting material may be deposited on or associated with the box, all of which must be penetrated by the clamp.
When used for purposes of an electrical grounding system, the clamp must be capable of conducting high amperage current to ground, as might occur in a lightning strike, without premature failure. For example, one standard test requirement in the telephone industry for a ground clamp which is intended to conduct a lightning strike to a ground rod is known as a "fusion current test." A length of No. 6 AWG solid copper wire is connected to the ground clamp, and 1,000 amperes electrical current is applied for a minimum of 20 seconds. To be acceptable, there must be no damage to the ground clamp and associated structure for the 20-second duration or until the No. 6 AWG solid copper wire fuses. The wire is not allowed to fuse within one inch of the clamp. Other current tests are known in other industries or standard setting or testing bodies, which involve passing sizable amounts of current through a solid copper conductor to the ground clamp for various time durations without damage to the clamp or the outlet box or conductor to which it is attached.
It has been known to propose a ground clamp having teeth resting against a hollow electrical utility box for grounding purposes. The spacing between the teeth is adjustable by a screw having break-off positions. However, such a mechanism is not capable of penetrating paint and other non-conductive surfaces which coat an electrical utility box in a manner to form an intimate ground connection, i.e., a ground connection sufficient to pass a fusion current test so that the ground clamp can be utilized for its intended purpose. Thus, there has been great need for an adjustable ground clamp which can span hollow utility boxes of various manufacturers and of various widths, and which form an intimate electrical contact for grounding purposes.